1. Field of the Invention
The invention relates to an optical filter that uses localized plasmons.
2. Description of the Related Art
A hole-type optical filter in which apertures are arranged periodically in a thin metal film and wavelength selection is performed using surface plasmons is described in U.S. Pat. No. 5,973,316 and Nature, Vol. 424, 14, Aug., 2003.
The transmissivity of a thin metal film having an aperture diameter of a size equal to or less than a light wavelength has been considered to be less than approximately 1%, with the specific value depending on the film thickness.
The description of U.S. Pat. No. 5,973,316 suggests that the transmissivity can be increased to a certain extent by using surface plasmons of the thin metal film surface.
Furthermore, Nature, Vol. 424 (P 824-830), 14, Aug., 2003 describes the possibility of obtaining a RGB transmission spectrum as a hole-type optical filter using surface plasmons.
In a conventional optical filter using a thin metal film, light transmissivity is typically about several percents. However, in an optical filter in which transmissivity is not that high, an incident light intensity sufficient to obtain a desired transmitted light intensity may be high. As a result, for example, the thin metal film may be heated as the incident light intensity rises, and structural changes may occur in the thin metal film. This can make it impossible to obtain the desired designed optical characteristics.
U.S. Pat. No. 5,973,316 discloses a feature for increasing the transmissivity by using a thin metal film structural body in which periodic apertures are provided in a thin metal film, and matching the arrangement period of the apertures with the wavelength of surface plasmons propagating in the thin metal film surface.
However, in the invention disclosed in U.S. Pat. No. 5,973,316, the aperture arrangement period generally has to be matched with the plasmon wavelength, and thus there is little freedom in designing optical characteristics. For example, it can be difficult to design an element having desired optical characteristics in a wide-zone wavelength range such as the entire visible range.
Nature, Vol. 424, 14, Aug., 2003 discloses the possibility of obtaining a RGB transmission spectrum as a hole-type optical filter, but investigations for increasing transmissivity and providing stable characteristics for such optical filters have not been conducted.
Accordingly, in the application of filters such as those described above, the stability of characteristics and endurance, in addition to optical characteristics such as transmissivity and wavelength zone, are in need of improvement.